The histopathologic changes caused by various methods of euthanasia were studied in rats, mice, guinea pigs and rabbits. Lesions resulting from a particular method of euthanasia were consistent from species to species. Each method studied affected the lungs to some degree, ranging from mild congestion of alveolar capillaries to marked intramural edema of pulmonary arteries and alteration of vascular permeability. Euthanasia of experimental animals by overexposure to CO2, or intraperitoneal injection of concentrated sodium pentobarbital seemed most suitable for pulmonary studies. Decapitation (mice, rats, guinea pigs), cervical dislocation (mice), CO2 and intracardial injection of sodium pentobarbital were more suitable for examination of abdominal viscera.